In recent years, memory systems comprising nonvolatile memories have been widely prevalent.
As such a memory system, a solid state drive (SSD) based on a NAND flash technology is known. The SSD is used as a storage for various computers in view of its features such as low power consumption and high performance.
Normally, the SSD is equipped with a large number of nonvolatile memory dies in order to increase the capacity of the SSD. The individual nonvolatile memory dies can operate independently. Thus, the nonvolatile memories can function as units for parallel processing.
However, typically, operations for one nonvolatile memory die are performed sequentially rather than in parallel. Thus, if a read request occurs for a nonvolatile memory die in which a write operation is being executed (i.e., die contention), a time until when the read request is responded to (i.e., read latency) may be very long.
Further, if data write is concentrated on some specific nonvolatile memory dies, the degrees of wear of the blocks in these specific nonvolatile memory dies increase, and consequently the life of the SSD may be shortened in some cases.